In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. Since numerous conductive features are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features (and close spacing between adjacent features) requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist.
The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation; that is, an image-wise exposure to radiation. This radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. Types of radiation commonly used in microlithographic processes include visible light, ultraviolet (UV) light and electron beam radiant energy. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist (depending upon whether a positive photoresist or a negative photoresist is utilized) resulting in a patterned or developed photoresist. Many developer solutions contain water and an active compound, such as water and a hydroxide compound.
Development is a critical step in photolithography. This is because poor development decreases resolution, precise pattern formation and/or the quality of subsequent processing steps. For example, incomplete development followed by an etching step results in an incompletely etched underlayer, which in the case of an incompletely etched metal layer, may lead to short circuits. Overdevelopment results in mal-formed structures which inhibit the proper function of semiconductor devices.
Poor development is caused by a number of factors. One factor is that microbubbles, present at the photoresist--developer interface, lead to pattern defects because they inhibit uniform attach of the photoresist by the developer. It is desirable for the developer to remove selected portions of the exposed photoresist uniformly across the wafer. Another factor is low wettability of the developer on the photoresist surface, which can lead to underdevelopment of the exposed photoresist, resulting in a poorly resolved pattern. Low wettability is attributable, in part, to the organic characteristics of the photoresist (typically but not always an organic polymeric material) and the aqueous characteristics of the developer (typically but not always an aqueous solution).
Lack of wettability of the developer on the photoresist surface also requires the undesirable use of large amounts of developer. Since increased amounts of developer increase the costs associated with photolithography, it is preferable to employ small amounts of developer (and consequently small amounts of the active developer chemicals), provided that highly resolved photoresist patterns are obtainable.